The expression “on-line generation” of data during a reaction is used herein to denote generation of the data sufficiently rapidly that the data is available essentially instantaneously for use during the reaction. The expression “generation of data in on-line fashion” during a reaction is used synonymously with the expression on-line generation of data during a reaction. Generation of data from a laboratory test (on at least one substance employed or generated in the reaction) is not considered “on-line generation” of data during the reaction, if the laboratory test consumes so much time that parameters of the reaction may change significantly during the test. It is contemplated that on-line generation of data can include the use of a previously generated database that may have been generated in any of a variety of ways including time-consuming laboratory tests.
With reference to a product being produced by a continuous reaction, the expression “instantaneous” value of a property of the product herein denotes the value of the property of the most recently produced quantity of the product. The most recently produced quantity typically undergoes mixing with previously produced quantities of the product before a mixture of the recently and previously produced product exits the reactor. In contrast, with reference to a product being produced by a continuous reaction, “average” (or “bed average”) value (at a time “T”) of a property herein denotes the value of the property of the product that exits the reactor at time T.
Throughout this disclosure, the expression “diluent” (or “condensable diluent” or “condensable diluent gas”) denotes condensable gas (or a mixture of condensable gases) present in a polymerization reactor with polymer resin being produced. The diluent is condensable at the temperatures encountered in the process heat exchanger. Examples of diluents include induced condensing agents (ICAs), comonomers, isomers of comonomers, and combinations thereof.
The expression “dry polymer resin” (or “dry version” of polymer resin) is used herein to denote polymer resin that does not contain substantial amounts of dissolved gas. An example of dry polymer resin is polymer that had been previously produced in a polymerization reactor and then purged to eliminate all (or substantially all) unreacted comonomers and ICAs that had been dissolved in the polymer at the time of production. As will be discussed herein, a dry version of polymer resin has significantly different melting behavior than would the same polymer resin if it were in the presence of a significant amount of condensable diluent gas and comonomer.
The expression polyethylene denotes a polymer of ethylene and optionally one or more C3-C10 α-olefins while the expression polyolefin denotes a polymer of one or more C2-C10 α-olefins.
Throughout this disclosure, the abbreviation “MI” denotes melt index.
One commonly used method for producing polymers is gas phase polymerization. A conventional gas phase fluidized bed reactor, during operation to produce polyolefins by polymerization, contains a fluidized dense-phase bed including a mixture of reaction gas, polymer (resin) particles, catalyst, and (optionally) catalyst modifiers. Typically, any of several process control variables can be controlled to cause the reaction product to have desired characteristics.
Generally in a gas-phase fluidized bed process for producing polymers from monomers, a gaseous stream containing one or more monomers is continuously passed through a fluidized bed under reactive conditions in the presence of a catalyst. This gaseous stream is withdrawn from the fluidized bed and recycled back into the reactor. Simultaneously, polymer product is withdrawn from the reactor and new monomer is added to replace the polymerized monomer. The recycled gas stream is heated in the reactor by the heat of polymerization. This heat is removed in another part of the cycle by a cooling system external to the reactor.
It is important to remove heat generated by the reaction in order to maintain the temperature of the resin and gaseous stream inside the reactor at a temperature below the polymer melting point and/or catalyst deactivation temperature. Further, heat removal is important to prevent excessive stickiness of polymer particles that if left unchecked, may result in loss of fluidization or agglomeration of the sticky particles which may lead to formation of chunks or sheets of polymer that cannot be removed as product. This phenomenon is commonly referred to as sheeting or chunking. Further, such chunks or sheets may fall onto the distributor plate causing impaired fluidization, and in many cases forcing a reactor shutdown. Prevention of such stickiness and/or sheeting has been accomplished by controlling the temperature of the fluid bed to a temperature below the fusion or sintering temperature of the polymer particles. Above this fusion or sintering temperature, empirical evidence suggests that such fusion or sintering leads to agglomeration or stickiness, which in turn can, if left unchecked, may lead to the above conditions including sheeting.
It is understood that the amount of polymer produced in a fluidized bed polymerization process is directly related to the amount of heat that can be withdrawn from the fluidized bed reaction zone since the exothermic heat generated by the reaction is directly proportional to the rate of polymer production. In steady state operation of the reaction process, the rate of heat removal from the fluidized bed must equal the rate of rate of heat generation, such that the bed temperature remains constant. Conventionally, heat has been removed from the fluidized bed by cooling the gas recycle stream in a heat exchanger external to the reactor.
A requirement of a fluidized bed process is that the velocity of the gaseous recycle stream be sufficient to maintain the reaction zone in a fluidized state. In a conventional fluidized bed polymerization process, the amount of fluid circulated to remove the heat of polymerization is greater than the amount of fluid required for support of the fluidized bed and for adequate mixing of the solids in the fluidized bed. The excess velocity provides additional gas flow to (and through) the fluid bed for additional cooling capacity and more intensive mixing of the reactor bed. However, to prevent excessive entrainment of solids in a gaseous stream withdrawn from the fluidized bed, the velocity of the gaseous stream must be regulated.
For a time, it was thought that the temperature of the gaseous stream external to the reactor, otherwise known as the recycle stream temperature, could not be decreased below the dew point of the recycle stream without causing problems of polymer agglomeration or plugging of the reactor system. The dew point of the recycle stream is that temperature at which liquid condensate first begins to form in the gaseous recycle stream. The dew point can be calculated knowing the gas composition and is thermodynamically defined using an equation of state.
A recycle stream can be cooled to a temperature below the dew point in a fluidized bed polymerization process resulting in condensing a portion of the recycle gas stream. The resulting stream containing entrained liquid is then returned to the reactor without causing agglomeration, and/or plugging phenomena. The process of purposefully condensing a portion of the recycle stream is known in the industry as “condensed mode” operation in a gas phase polymerization process.
When a recycle stream temperature is lowered to a point below its dew point in “condensed mode” operation, an increase in polymer production is possible, as compared to production in a non-condensing mode because of increased cooling capacity. Consequently, a substantial increase in space-time yield, the amount of polymer production in a given reactor volume, can be achieved by condensed mode operation with little or no change in product properties.
Cooling of the recycle stream to a temperature below the gas dew point temperature produces a two-phase gas/liquid mixture with solids contained in both of these phases. The liquid phase of this two-phase gas/liquid mixture in “condensed mode” operation remains entrained or suspended in the gas phase of the mixture. Vaporization of the liquid occurs only when heat is added or pressure is reduced. In some conventional processes, vaporization occurs when the two-phase mixture enters the fluidized bed, with the (warmer) resin providing the required heat of vaporization. The vaporization thus provides an additional means of extracting heat of reaction from the fluidized bed. The heat removal capacity is further enhanced in condensed mode operation by the lower gas temperatures of the gas stream entering the fluidized bed. Both of these factors increase the overall heat removal capability of the system and thereby enable higher space-time yields (higher reactor production rates per unit volume of the fluidized bed).
The cooling capacity of recycle gas can be increased further while at a given reaction temperature and a given temperature of the cooling heat transfer medium. One option described is to add non-polymerizing, non-reactive materials to the reactor, which are condensable at the temperatures encountered in the process heat exchanger. Such non-reactive, condensable materials are collectively known as induced condensing agents (ICAs). Increasing concentrations of ICA in the reactor causes corresponding increases in the dew point temperature of the reactor gas, which promotes higher levels of condensing for higher (heat transfer limited) production rates from the reactor. Suitable ICA materials are selected based on their specific heat and boiling point properties. In particular, an ICA compound is selected such that a relatively high portion of the material is condensed at the cooling water temperatures available in polymer production plants, which are typically 20-40° C. ICA materials include hexane, isohexane, pentane, isopentane, butane, isobutane and other hydrocarbon compounds that are similarly non-reactive in the polymerization process.
U.S. Pat. No. 5,352,749, to DeChellis et al, teaches that there are limits to the concentrations of condensable gases, whether ICA materials, comonomers or combinations thereof, that can be tolerated in the reaction system. Above certain limiting concentrations, the condensable gases can cause a sudden loss of fluidization in the reactor, and a consequent loss in ability to control the temperature in the fluid bed. The above-cited U.S. Pat. No. 5,352,749, and U.S. Pat. Nos. 5,405,922 and 5,436,304, disclose upper limits of ICA in the reactor, depending on the type of polymer being produced. U.S. Pat. No. 5,352,749 discloses that a limiting concentration of ICA (isopentane) exists, beyond which the reactor contents suddenly loose fluidization. The authors characterized this limit by tracking the ratio of fluidized bulk density to settled bulk density. As the concentration of isopentane was increased, they found that the bulk density ratio steadily decreased. When the concentration of isopentane was sufficiently high, corresponding to a bulk density ratio of 0.59, they found that fluidization in the reactor was lost. They therefore determined that this ratio (0.59) was a point of no return, below which the reactor will cease functioning due to loss of fluidization.
As described in PCT Application Publication Number WO 2005/113615(A2), attempts to operate polymerization reactors with excessive ICA concentrations cause polymer particles suspended in the fluid bed to become cohesive or “sticky,” and in some cases cause the fluid bed to solidify in the form of a large chunk. This stickiness problem is characterized by undesirable changes in fluidization and mixing in the fluid bed, which if left unchecked, may develop into a reactor discontinuity event, such as sheeting in the straight sided reaction section, sheeting in the dome of such a reactor or chunking, any of which can lead to reactor shut-downs, which in large scale reactors are expensive. These solid masses (sheets or chunks) of polymer eventually become dislodged from the walls and fall into the reaction section and settle on the distributor plate, where they interfere with fluidization, block the product discharge port, and usually force a reactor shut-down for cleaning. The term “discontinuity event” is used to describe a disruption in the continuous operation of a polymerization reactor caused by sheeting, chunking or distributor plate fouling. The terms “sheeting and/or chunking” while used synonymously herein, may describe different manifestations of problems caused by excessive polymer stickiness in the fluid bed. In either manifestation (sheeting or chunking) the excessive polymer stickiness can lead directly to a reactor discontinuity event with the associated loss production.
Desirable, smooth operation of a fluidized bed polymerization reactor system is generally characterized by efficient, random mixing of particles in the fluidized bed. Undesirable operation is typically characterized by non-uniformities, including “hot spots,” “cold bands” (caused by insulating layers of fines), and polymer agglomerates (e.g., chunks or sheets) in the reactor.
The parent application (published as U.S. Patent Application Publication Number US 2003/0121330 A1) and corresponding PCT Application Publication Number WO 03/051929 describe use of mathematical chaos theory to detect the onset and presence of sheeting in a fluidized bed reactor, and teaches generating time series data from the output of a range of instruments, including acoustic emission sensors, fluidized bulk density sensors, differential pressure sensors, static sensors, and wall temperature sensors, processing the data in accordance with methods of non-linear dynamics herein referred to as chaos theory and comparing the resulting processed data to data from a control reactor running without sheeting. In some embodiments disclosed therein, the onset of sheeting is indicated by an increase in mean “cycle time” associated with the time series (relative to mean cycle time in a baseline, control reaction), usually with a concurrent decrease in the “mean deviation” of the time series, or by a decrease in mathematical “entropy” (e.g., Kolmogorov entropy or Shannon entropy) of the time series data as compared to the entropy of time series data from a control reaction running without sheeting. FIG. 10 of the parent application is a plot of Kolmogorov entropy of each of a number of sub-sequences of a time series of fluidized bulk density data values (resulting from measurements during a polymerization reaction) versus time. FIG. 9 of the parent application is a plot of modified Shannon entropy of data (measured during a polymerization reaction) versus time.
It has been proposed in the literature to measure data from fluidized bed (and other) reactors in contexts other than during performance of a resin polymerization reaction in any such reactor, to generate Kolmogorov entropy (or other mathematical entropy) values from the measured data and to determine from the entropy values some characteristic of operation of the reactors. For example, Finney, et al., in the paper “Measuring Slugging Bed Dynamics with Acoustic Sensors,” submitted to KONA: Powder and Particle (1997), teach determining Kolmogorov entropy of acoustic data and DP (differential pressure) data and using Kolmogorov entropy of acoustic data to predict or detect “slugging” in a fluidized bed.
However, until the present invention it had not been known how to generate reaction parameter data in on-line fashion by measuring at least one parameter of a polymerization reaction in a fluidized bed reactor, and to generate Kolmogorov entropy (or other mathematical entropy) values from the measured reaction parameter data and to use the entropy values as an indicator of the degree (or imminence) of polymer resin stickiness or of approach to unsafe polymerization reactor operating conditions leading to at least one of sheeting and chunking.
It would be desirable to provide a method of determining a stable operating condition for fluidized bed polymerization, especially if operating in condensed mode, to facilitate optimum design of the plant and determination of desirable process conditions for optimum or maximum production rates for a given plant design.
It would also be desirable to have a mechanism in commercial gas-phase polymerization reactors to detect the onset of stickiness that is a better or earlier indicator of the onset of stickiness than are conventional techniques (e.g., monitoring the fluidized bulk density as described in U.S. Pat. No. 5,352,749). Such a mechanism would allow the operators to determine when conditions of limiting stickiness are being approached, and enable them to take corrective action before discontinuity events (such as sheeting and chunking) occur, while keeping the reactors at or near conditions that permit higher production rates with substantially less risk. In a class of embodiments, the present invention provides a better indicator of onset of resin stickiness (and of degree of resin stickiness and of imminence of unsafe polymerization reactor operating conditions) than do conventional techniques, using Kolmogorov entropy or other mathematical entropy values determined from the measured polymerization reaction parameter data.
Polymerization reaction parameter data to be processed in accordance with some embodiments of the present invention are indicative of a sequence of static charge values. Such data can be generated by monitoring a polymerization reaction using one or more static probes in any of a variety of conventional ways (e.g., as described in US Patent Application Publication No. 2005/0148742, published Jul. 7, 2005). US Patent Application Publication No. 2005/0148742 describes use of static probes positioned in the entrainment zone of a fluidized bed polymerization reaction system to monitor “carryover static” during a polymer resin-producing polymerization reaction in the reactor system, and describes control of the reaction in response to the results of such monitoring to prevent discontinuity events such as chunking and sheeting (e.g., to reduce carryover static and thereby prevent such discontinuity events). The expression “entrainment zone” of a fluidized bed reactor system is used in US Patent Application Publication No. 2005/0148742 and the present disclosure to denote any location in the reactor system outside the dense phase zone of the system (i.e., outside the fluidized bed). However, US Patent Application Publication No. 2005/0148742 does not suggest wavelet transforming static charge data (or other data) or determining the kurtosis of a set of static charge data values (or other data values).
The expression “carryover static” is used in US Application Publication No. 2005/0148742 and the present disclosure to denote static charging that results from frictional contact by particles (e.g., catalyst particles and resin particles) against the metal walls of a gas recycle line, or against other metal components in a reactor entrainment zone. Carryover static can be measured by suitable static probes positioned in various sections of the entrainment zone of a reaction system, including the expanded (disengagement) section, the recycle line, and the distributor plate.
In the present disclosure, the expression “entrainment static” denotes carryover static that results from frictional contact between entrained particles and a static probe located in a gas recycle line of a fluidized bed reactor system. Thus, the term “entrainment static” represents a specific means of measuring the carryover static generated by frictional contact of entrained particles that occur throughout the gas recycle system.